1. Field of the Invention
The present invention relates to an electro-optic probe, which is used for observing the waveforms of a test signal based on a change in the polarization state of a light pulse caused when the light pulse generated by a timing signal is input into an electro-optic crystal which is coupled with an electric field generated by the test measuring signal, and particularly relates to an improved probe for electro-optic sampling oscilloscope.
2. Background Art
An electro-optic probe is capable of observing waveforms of a test signal based on a change in the polarization state of a laser light caused when the light pulse generated by a timing signal is input into an electro-optic crystal which is coupled with an electric field generated by the test measuring signal. When the laser light is emitted in a pulsed mode, and when the test signal is used after sampling, a measurement can be executed that has a very high time resolution. An electro-optic sampling oscilloscope using an electro-optic probe is developed based on the above phenomenon.
The electro-optic sampling oscilloscope (hereinafter, called EOS oscilloscope) has following advantages over the conventional sampling oscilloscope using electric probes:
(1) Measurement is easy, because the ground line is not necessary during measurement.
(2) Since the metal pin disposed at the distal end of the electro-optic probe is insulated from the measuring circuit, a high input impedance is provided, which results in eliminating factors that disturb the conditions of the test point.
(3) The use of the light pulse allows carrying out wide band measurement reaching to the GHz order.
The structure of the conventional electro-optic probe used in the measurement of signals by the EOS oscilloscope will be described with reference to FIG. 2. The reference numeral 1 denotes a probe head made of an insulator and a metal pin 1a is inserted in the probe head 1. The reference numeral 2 denotes an electro-optic element, and a reflecting film 2a is formed on one end surface of the electro-optic element which the metal pin 1a contacts. The reference numeral 5 denotes a quarter-wave plate. The numerals 6 and 8 denote polarization beam splitters. The numeral 7 denotes a Faraday element. The numeral 9 denotes a collimating lens, and the numeral 10 denotes a laser diode which emits laser light in accordance with pulse signals.
The numerals 11 and 13 denote condenser lenses, and 12 and 14 denote photodiodes which output electric signals to the EOS oscilloscope after converting the input laser beam into electric signals. The numeral 15 denotes the probe body. The numeral 17 denotes an isolator constituted by the quarter-wave plate 5, two polarization beam splitters 6 and 8, and the Faraday element 7. The isolator is used for transmitting the output light from the laser diode 11, and for separating the reflected light by the reflecting film 2a. 
Next, the optical path of the laser light emitted from the laser diode 10 will be described with reference to FIG. 2. The path of the laser light is represented by the reference symbol A.
The laser beam emitted from the laser diode 10 is converted into a parallel beam by the collimating lens 9, and input into the electro-optic element 2, after rectilinearly advancing through the polarization beam splitter 8, the Faraday element 7, and the polarizing beam splitter 6, and further passing the quarter-wave plate 5. The light beam input into the electro-optic element 2 is reflected by the reflecting film 2a formed at the end surface of the electro-optic element 2 facing to the metal pin 1a. 
The reflected laser beam passes the quarter-wave plate 5, and a part of the laser light beam enters the photodiode 12 after being reflected by the polarized beam splitter 6 and being condensed by the condenser lens 11. The laser beam, which passes the polarizing beam splitter 6, enters into the photodiode 14, after being reflected by the polarization beam splitter 8 and condensed by the condenser lens 13.
The quarter-wave plate is used for adjusting intensities of two laser beams entering into two photodiodes 12 and 14 becomes identical.
Hereinafter, the operation of measurement by the use of the electro-optic probe shown in FIG. 2 is described.
When the metal pin 1a contacts with a measuring point, a voltage is generated at the metal pin 1a and the voltage propagates to the electro-optic element 2, which results in causing a change of the refractive index of the electro-optic element due to the Pockels effect. After the optical property of the electro-optic element changes, and when the laser beam emitted by the laser diode 10 enters and propagates through the electro-optic element, the polarization state of the electro-optic element changes. The laser beam having the thus changed polarization state is, after being reflected by the reflecting 2a, condensed and enters into the photodiodes 12 and 14 and is converted into electric signals.
The change of the voltage applied to the measuring point is reflected as the change of the polarization state of the laser light by the electro-optic element 2, and the change of the polarization state is detected by the difference between the output from the photodiodes 12 and 14. Thus, by detecting the output difference, an electric signal applied to the metal pin 1a can be measured.
The change of the polarizing state of the electro-optic element caused by the change of the voltage applied to the measuring point can be detected as the difference of the polarizing state between two photodiodes 12 and 14. Thus, the electric signal applied to the metal pin 1a can be measured by detecting the output difference.
Since the EOS oscilloscope shown in FIG. 2 usually detects the change of refractive index of the elctro-optic element, it is preferable to eliminate other factors affecting on the change of the refractive index of the electro-optic element in addition to the voltage applied to the metal pin la, because such factors will cause errors in the test measurement.
However, there is a problem that if the temperature of the electro-optic element changes, the refractive index of the electro-optic element changes so that accurate measurements cannot be made.
It might be possible to carry out measurement by compensating the temperature change in signal processing, but another problem that arises is that the temperature dependence of the electro-optic element is nonlinear.
It is therefore the object of the present invention to provide an electro-optic probe for EOS sampling oscilloscope which is capable of improving the accuracy of the measurement by maintaining the temperature of the electro-optic element at constant.
According to the first aspect of the present invention, there is provided a probe for an electro-optic sampling oscilloscope comprising: a laser diode for emitting a laser beam based on a control pulse signal of a body of the electro-optic sampling oscilloscope; a collimating lens for collimating said laser beam; an electro-optic element having a reflecting film at one end and the optical property thereof that change due to an electric field propagated through a metal pin provided at the end surface on the reflecting film side; an isolator, provided between said collimate lens and said electro-optic element, for separating the laser beam reflected by said reflecting film after passing the laser beam emitted by said laser diode; and photodiodes for converting the reflected light separated by said isolator into electric signals; wherein, the probe for the electro-optic sampling oscilloscope further comprises: a temperature detecting portion disposed so as to abut said electro-optic element for detecting and outputting a temperature of the electro-optic element; a temperature adjusting portion, disposed so as to face said temperature detecting portion abutting said electro-optic element, for adjusting the temperature of said electro-optic element in response to the detected temperature; and a temperature controlling portion for controlling said temperature adjusting portion based on said detected temperature.
According to the second aspect of the present invention, in a probe for an electro-optic sampling oscilloscope according to the first aspect, said temperature detecting portion is formed by a thermister and the temperature of said electro-optic element is detected by the change of the resistance.
According to the third aspect, in a probe for an electro-optic sampling oscilloscope according to the second aspect, said temperature adjusting portion is formed by a heater, and the temperature of said electro-optic element is increased by changing the electric current flowing in said heater.
According to the fourth aspect, in a probe for an electro-optic sampling oscilloscope according to the first aspect, said temperature adjusting portion is formed by a Peltier element and the temperature of the electro-optic element is increased or decreased by changing the current flowing in said Peltier element.
According to the fifth aspect, in a probe for an electro-optic sampling oscilloscope according to the first aspect, said temperature controlling portion controls the temperature adjusting portion based on the detected results of said temperature detecting portion such that the temperature of said electro-optic element is maintained at a temperature where the temperature characteristic of said electro-optic element is excellent.